Ultrahigh-frequency electro-magnetic wave transmission apparatus



y 1952 R. LATHAM ET AL 3,032,727

ULTRAHIGH-FREQUENCY ELECTROMAGNETIC WAVE TRANSMISSION APPARATUS FiledMay 2, 1960 Fig. 00 PRIOR ART P F W60 WWW:

70mm Maw/mm;

BY wwa 7 United States Patent Qfifice 3,032,727 ULTRAHIGH-FREQUENCYELECTRO-MAGNETIC WAVE TRANSMISSION APPARATUS Robert Latham, London, andFrederick Charles Thompson, Danbury, England, Herbert Sixsmith, Boulder,

Colo., and Maurice Esterson, Risings, Great Baddow, Andrzej Folkierski,London, and Alan Hugh Pickering, Galleywood, Chelmsford, England,assignors to English Electric Valve Company Limited, London, England, aBritish company Filed May 2, 1960, Ser. No. 25,994 3 Claims. (Cl.333-98) This invention relates to ultrahigh-frequency electromagneticwave apparatus and is more particularly concerned with apparatus of thekind in which ultrahighfrequency electro-magnetic wave energy isrequired to be transmitted out of, or into, a hermetically sealed space.

This application is a continuation-in-part of our applica tion SerialNo. 461,896, filed October 12, 1954, now abandoned.

In any apparatus in which ultrahigh-frequency electromagnetic waveenergy is required to be fed through the enclosing wall of ahermetically sealed space, a dielectric window is provided in the wallto maintain the hermetic seal while allowing the waves to passthroughthe window. Thus, in apparatus in which centimeter waves aregenerated in the evacuated envelope of a magnetron or of a klystron, itis usual to take out the energy either by means of a coaxial line, thesupporting dielectric of which acts as a window and is made part of theevacuated envelope of the tube, or by means of a wave guide with avacuumtight window sealed in to form part of the tube envelope. Thecoaxial line type of arrangement has severe output limitations set bythe danger of electric breakdown, especially at higher frequencies. Thewave guide type of output is, therefore, much preferred at higher outputpowers, but this also has important limitations which will be describedlater herein, and which it is the object of the present invention toavoid.

The invention is illustrated in and further explained in connection withthe accompanying schematic drawings in which:

FIG. 1 is a schematic showing of a known arrangement; and

FIGS. 2 and 3 are schematic representations of typical embodiments ofthe present invention.

Referring to FIG. 1, a wave guide shown therein consists of an inputportion WGI in line with an output portion WGO. Wave power travels alongthe wave guide and its direction of propagation is indicated by thearrows marked P. Between the input and output portions. there is adielectric window DW surrounded by choke ditches CD and hermeticallysealed in vacuum-tight manner in a metal frame member FM which is shownbroken away and which forms part of the wall of an evacuated chamber(e.g. the envelope of a discharge tube not otherwise shown), in whichchamber the input portion WGI is positioned.

With this construction, since the plane of the window DW is at rightangles to the run of the wave guide, the said window extending across anaperture through which the power is transmitted, the electric field is,as indicated by the arrows F, substantially parallel to the surface ofthe dielectric window DW. With this type of window the fraction of powerlost in the window cannot be substantially reduced by varying thedetails of the construction within the general plan. Apart, therefore,'from considerations of actual breakdown across the surface of thewindow, the power handling capabilities are limited by dielectric lossin the material of which the window is made. Accordingly, once thislimitation is reached, the only way of increasing the power transmittedis to increase the size of the wave guide and the area of the window butthis introduces other difficulties. For example, it becomes moredifficult effectively to cool the window and the larger wave guide makesinterfering modes possible.

One proposed solution of this problem consists in having at least thecenter portion of the dielectric window made of a much more refractorymaterial than glass so that the window may withstand highertemperatures. The power loss in such a window, however, remains high andfurthermore this arrangement is costly and critical to manufacture.

Another proposed solution consists in employing a hollow conical windowmounted in a length of circular wave guide and having a length of atleast half a wavelength at the operating frequency. This arrangementalthough effective to reduce the temperatures of the window, is costlyto manufacture due to the special shape of the window and furthermorerequires the provision of rectangular-to-circular transition wave guidesections where the device is to be used in conjunction with rectangularinput and output wave guides as is commonly the case.

According to the present invention, a high power VHCl.l-

um-tight coupling comprises an input wave guide, an output wave guidehaving its axis parallel to and off-setfrom the axis of said input waveguide, said wave guides overlapping and terminating thereat andconnecting section-means for coupling substantially all the wave energyin said input wave guide to said output wave guide comprising a sectionconnected to the overlapping portions of said wave guides, ahermetically sealed window positioned between said wave guides in saidsection at their common junction, said window being so disposed that itseffective surface is substantially perpendicular to the direction of theelectric field of wave energy propagated in said wave guides in thedominant mode, and extending in a plane which is in spaced parallelrelation to both said waveguides, said dielectric material providing theonly wave energy coupling path between said wave guides. With regard tothe modes of wave propagation in the arrangements in accordance withthis invention, it is pointed out that in all cases the dominant mode ofpropagation is the important one. In the case of rectangular waveguides. this mode is the H and in circular wave guides It is the H11.

In FIG. 2, which shows one embodiment of the invention, two wave guideportions WGI and WGO are arranged to overlap, as shown. The adjacentguide walls at the region of the overlap may be, as shown, connected bya short section sealed to the waveguides. A wave energy path from WGI toWGO is provided by a vacuumti ht dielectric window DW encircled by chokeditches CD in the connecting section, the surface of the window DW beingsubstantially perpendicular to the electric field direction of theenergy propagated in the wave guide, which in this arrangement is in theH mode of propagation. This is indicated by the arrows F, the'arrows Prepresenting the power flow. With this arrangement the fraction of powerlost is considerably less than with the previous design described above,if the same dielectric material is used in each case. The choke ditchesare not essential since direct sealing of the window without ditches maybe employed. but the provision of the said ditches is preferred asmaking for easier assembly.

FIG. 3 shows a further modification in which the inlet wave guide WGI issuperimposed over the outlet wave guide WGO with a plane dielectricwindow DW therebetween. This arrangement is suitable for rectangularwave guides in which the H mode of propagation takes place. the windowbeing perpendicular to the electric field direction P and beingsurrounded by choke ditches CD. The outlet wave guide WGO, as shown,initially doubles Patented May 1, 1962 3 back on the inlet wave guideWGI. The arrows P in FIG. 3 again indicate power flow.

Arrangements in accordance with this invention are capable of handlingmuch more power for a given size than known arrangements as typified byFIG. 1, and by providing suitable irises (not shown) as known per se,for impedance matching, can be arranged to fulfil the requirements for asatisfactory microwave window.

The ease of assembly of the structure will now be described in detail.Where it is required to braze metal, such as at the junction ofwaveguides WGI and WGO, it is an advantage to have the glass-to-metalseal spaced away from the heat which is normally generated at thebrazing point. In the case of the embodiments shown in FIGS. 2 and},there isshown between the glass window and the outer part of thewaveguide a metal member which spaces the window and its metal-to-glassseal from the envelope itself. In order'that there may be an electricalshort-circuit between the wave guides, the ditches are inserted. Theparticular advantage is, of course, that the metal between the envelopeand the glass window can be of the requisite material for enabling anefiective metal-to-glass seal to be carried out at the point of contactwith the glass.

While we have described our invention in certain preferred embodiments,we realize that modifications may be made and we desire that it beunderstood that no limitations upon ourinvention are intendedother thanmay be imposed by the scope of the appended claims.

We claim:

1. A high-power vacuum-tight coupling comprising an input wave guide, anoutput wave guide having its axis parallel to and spaced from said inputwave guide, said wave guides overlapping and terminating thereat andconnecting section means for coupling substantially all the wave energyin said input wave guide to said output wave guide comprising a sectionconnected to the overlapping portions of said waveguides, a hermeticallysealed window positioned between said wave guides at their commonjunction, said window being so disposed that its effective surface issubstantially perpendicular to the direction of the electric field ofwave energy propagated in said Wave guides in the dominant mode, andextending in a plane which is in spaced parallel relation to both saidwaveguides, said dielectric material providing the only wave energycoupling path between said wave guides, said window being off-set fromthe surfaces of both of said guides and a choke ditch encircling thewindow on each side thereof.

'2. A coupling as claimed in claim 1 wherein said input and output waveguides extend away from said connecting section means in oppositedirections.

3. A coupling'as claimed in claim l wherein said output wave guideinitially doubles back on said input wave guide and then curves awayfrom said input wave guide, the arrangement being such that the electricfield energy propagated therethrough is turned through substantially aright angle with respect to the direction of the electric field in theinput channel.

References Cited in the file of this patent UNITED STATES PATENTS

